ADDRESS. | lxi 
in one mode of interaction, may become the effect, when the order of the 
phenomena is changed ;—and that therefore, in the words of Mr. Grove, one 
of the able expounders of these views, while they are “correlative” or reci- 
procally dependent, “neither, taken abstractedly, can be said to be the 
essential cause of the others.” 
But a further step remained to be taken. If these forces were not only 
related, but mutually related, was it not probable that the relation was alsoa 
_ definite one? Thus, when heat is developed by mechanical action, ought we 
not to expect a certain definite proportion to subsist between the interacting 
forces, so that if one were doubled or trebled in amount, the other should 
undergo a proportionate change? This anticipation, it has been already 
stated, has been realized by Mayer and Joule. The discovery of the mecha- 
nical equivalent of heat has been rapidly followed by that of other forces ; 
and we now know not only that electricity, magnetism, and chemical action, 
in given quantities, will produce each a definite amount of mechanical work, 
but we know further—chiefly through the labours of Mr. Joule—what»that 
relation is, or, in other words, the mechanical equivalent of each force. 
The first step in this important career of discovery—though long unper- 
ceived in its relation to the rest—was, undoubtedly, Faraday’s proof of the 
definite chemical effect of the voltaic current. The last will probably be to 
reduce all these phenomena to modes of motion, and to apply to them the 
known principles of dynamics, in such a way as not only to express_the laws 
of each kind of movement, as it is in itself, but also the connexion and de- 
pendence of the different classes of the phenomena. 
A bold attempt at such a generalization has been made by M. Helmholtz. 
The science of Thermo-dynamics starts from the principle, that perpetual 
motion is impossible, or, in other words, that we cannot, by any combination 
of natural bodies, produce force out of nothing. In mechanical force, this 
principle is reducible to the known law of the conservation of living force ; 
and M. Helmholtz has accordingly endeavoured to show that this law is 
- maintained in the interaction of all the natural forces; while, at the same 
time, the assumption of its truth leads to some new consequences in physics, 
not yet experimentally confirmed. Expressed in its most general form, this 
principle asserts that the gain of vis viva during the motion of a system, is 
equal to the force consumed in producing it; from which it follows, that the 
sum of the vires vive, and of the existing forces, is constant. This principle 
M. Helmholtz denominates the conservation of force. A very important 
consequence of its establishment must be, that all the actions of nature are 
due to attractive and repulsive forces, whose intensity is a function of the 
distance,—the conservation of vis viva holding only for such forces. 
It is usually stated, in mechanical works, that there is a loss of vis viva in 
the collision of inelastic bodies, and in friction. This is true with respect to 
the motion of masses, which forms the subject of mechanical science as at 
present limited; but it is not true in a larger sense. In these, and such like 
